Disc drives are data storage devices used to store and retrieve digital user data in a fast and efficient manner. A typical disc drive stores such data on one or more rigid magnetic recording discs which are rotated at a constant high speed. An actuator controllably moves a corresponding number of data transducing heads to access data stored in tracks defined on the disc surfaces.
Servo data are written to the discs during disc drive manufacturing to define the tracks and to provide head positional information to a closed loop servo control circuit. The servo data are transduced and used by the servo control circuit during both seeking and track following operations.
The servo data include index data used to identify an index reference position on the disc surfaces. The index reference position corresponds to a “once-around” (i.e., zero degrees) angular reference for the discs. A typical servo control circuit tracks the angular position of the head by counting the number of servo fields encountered after each occurrence of the index reference position.
Because of track-to-track phase incoherence, a servo control circuit can misidentify the occurrence of the index reference position as the head is swept past the tracks during a high velocity seek operation. An index frame can be misdetected as a non-index frame, and vice versa. Such a false index detection can cause disorientation of the servo circuit (i.e., a loss of synchronization with the actual frame count), leading to uncertainty in the angular position of the head with respect to the disc.
Accordingly, there remains a continued need for improved approaches to maintaining an accurate indication of the angular location of a selected head during and after seek operations in a disc drive, and to eliminate unnecessary corrective actions (including full servo reinitialization operations) when spurious index servo signals are transduced during a seek.